Looks like those rumours of a new Siren game may be just that: rumors. As the creator of the franchise Keiichiro Toyama isn’t aware of the supposid new game. A fan inquired on twitter about the rumors of a new Siren game to which Toyama responded with:
Now Toyama could of could indeed be working on a new Siren and and just isn’t allowed to talk about it, but as it stands right now it looks like this rumors are bunk. That or maybe the new Siren just isn’t being made by Toyama and his team. Really hope it’s the former, because I’d love to see Toyama and his team make another game similar to Siren: Blood Curse.
[Source]